(Hypertension. 2001;38:38.)
© 2001 American Heart Association, Inc.
Editorial Commentary |
Peaks and Valleys
From the Franz Volhard Clinic and Max Delbrück Center for Molecular Medicine, Medical Faculty of the Charité, Humboldt University of Berlin, and INFOGEN, Valigen N.V., Berlin-Buch, Germany
Correspondence to Friedrich C. Luft, Wiltberg Strasse 50, 13125 Berlin, Germany. E-mail luft{at}fvk-berlin.de
Key Words: genes • blood pressure • exercise • linkage
In this issue of Hypertension, Rankinen et al1 report on a genome scan for gene loci related to exercise-induced blood pressure elevations from the HERITAGE Family Study. The investigators studied 344 sibling pairs from 99 white nuclear families and 102 sibling pairs from 104 black nuclear families. The hypothesis was that linkage (the question, "where is it?") could be found between certain gene loci and blood pressure elevations at baseline, after acute exercise, and after exercise training. The subjects were tested at 50 watts (tooth brushing equivalent) and 80% of VO2 maximum (fairly hard work). The subjects underwent an exercise training program involving 30-minute sessions, which were gradually increased to 50 minutes, 3 times weekly for at least 6 weeks. Thereafter, the subjects were retested, and the difference (
) values were tested as phenotypes to examine a training effect.
See p 30
The authors are to be congratulated for tackling a highly clinically relevant hypothesis in a search for new genes related to blood pressure changes with fitness. Their data (decrease in maximal blood pressure in response to exercise 50 W) showed that the training responses were profound. The family fathers showed a 16 mm Hg drop in systolic blood pressure and a 7 mm Hg drop in diastolic blood pressure with training. The mothers’ drops in blood pressure were almost as good and exhibited similar impressive changes. The children were decidedly less responsive, although they also showed a significant decrease in exercise-induced blood pressure. Presumably, the children were more fit at baseline. At the higher workload, the results were less impressive. Heart rate, also an interesting parameter of fitness, was not examined in this study. Gene loci for heart rate have been identified in the rat and would be an interesting subject of investigation.2
The authors performed a total genome scan with 344 microsatellite markers. "Promising" linkage (lod 
1.75) was detected for baseline blood pressure at various places along the genome. These loci differed between white and black families. Suggestive evidence for linkage was found for both groups at other areas in the genome. Some of the areas indicating possible linkage enclose plausible, already known candidate genes. The authors found the strongest evidence for linkage on chromosome 8, where white families scored a lod of 2.36 in terms of linkage to the phenotype response to training at 50 W. Black families showed no evidence for linkage between this locus and the 50 W phenotype. The genes for 11ß-hydroxylase and aldosterone synthase happen to reside at this locus on chromosome 8. These genes are already known to us through the work on monogenic forms of hypertension.3
The chances of cloning the responsible gene (the question, "what is it?") are high in monogenic conditions. For complex genetic diseases, however, problems with cloning achieve another dimension entirely. What is the likelihood that the HERITAGE Family Study and numerous similar endeavors will lead us to new genes responsible for hypertension or blood pressure regulation? Some of the investigators of the present study recently reported the Québec Family Study.4 In that study, 125 random and 81 obese families were examined. Resting blood pressure was the phenotype. A total of 420 markers were examined in a total genome scan. Chromosomal areas on 3q, 10p, 12p, 14q, and 22q were identified as suggesting linkage. The genes for hydroxysteroid beta hydroxylase, angiotensinogen, angiotensinogen-converting enzyme, and adipsin lie within certain of these areas. Two other recent total genome scans for blood pressure were recently reported in Hypertension. Pankow et al5 indicate that there may be 1 gene on chromosome 18q that regulates systolic blood pressure during the physiological recovery period after a postural stressor. They studied blood pressure responses to a postural change and appear to have confirmed an earlier mapping study of an "orthostatic hypotension" kindred.6 Their multipoint lod score achieved a value of 2.6. Levy et al7 studied families from the Framingham cohort. They had the advantage of being able to study longitudinal blood pressure values and found a locus on chromosome 17 that yielded a multipoint lod score of 4.7. For diastolic blood pressure, the lod value was 2.0. This locus is of interest because of earlier animal studies showing linkage at a site syntenic to this locus.8
Another approach to linkage is to study siblings who are extremely divergent for blood pressure. This approach was used in studies of children from Rochester, Minn,9 and Chinese adults.10 Rather than the sharing of marker alleles, the divergent siblings would be expected to not share marker alleles at the loci in question. In the Chinese study, a maximum lod of 3.77 was achieved for a locus on chromosome 15.11 When genes reside at linkage loci that are familiar to us, we are pleased. The angiotensinogen gene locus, the ß2-adrenergic receptor gene locus, and genes related to sodium reabsorption show up with reassuring regularity. After all, we have candidate genes in abundance.12 More difficult to answer is whether or not the goal of finding new genes is to be realized.
How are the cloning strategies for complex genetic diseases to be performed, and how is our track record? Linkage studies involving loci of candidate genes that can then be investigated with an association approach are of proven utility.13 When no candidate gene is available, however, further fine mapping is necessary by studying still more families or sibling pairs. Furthermore, linkage dysequilibrium mapping can be utilized. Boerwinkel et al14 recently presented a brief review on the basic strategy. The method relies on single nucleotide (bi-allelic) polymorphisms (SNPs), mutations that are distributed approximately every 1000 base-pairs across the genome. The SNPs represent interindividual variability that distinguishes 1 person from another. Probably more than 3 000 000 SNPs reside within the genome. With the results of the human genome project soon to be available, investigators will be able to analyze all the SNPs that are likely to be informative. SNPs in coding, noncoding, and regulatory regions are all valuable. The SNPs can be used in haplotype analyses to determine if specific haplotypes are associated with the phenotype. Once SNP mapping is completed, the gene or genes must be sequenced for mutations. Finally, experimental animal or cellular studies on the gene in question will be necessary to prove how the mutation works.
Previously, skeptics (such as the authors) have dourly observed that no new gene has been cloned for any complex genetic disease on the basis of a linkage analysis. Linkage studies of candidate genes do not count in our view. That angiotensinogen might be related to hypertension or apolipoprotein E (APOE) might be related to atherosclerosis comes as no great surprise. We may have been too harsh on APOE because that gene locus was linked to Alzheimer’s disease, which was not necessarily expected, and variants in that gene exert a strong influence on the possibility of contracting the condition at an earlier age.15 The APOE locus was also linked to the disease solely on the basis of a SNP analysis, as a proof of principle.16 Recently, the gene for Cd36, a fatty acid transporter, was cloned in spontaneously hypertensive rats (SHR) on the basis of a linkage analysis with help from a gene expression study.17 Cd36 has more to do with lipid metabolism than with blood pressure. Congenic experiments have supported the role of a Cd36 mutation in the dyslipidemia featured by SHR.18
Very recently, Horikawa et al19 showed that the gene (CAPN10) for the protein calpain-10 is associated with type 2 diabetes mellitus. This study was a cloning effort that followed a linkage analysis performed in Mexican-American subjects from Starr County, Tex.20 The linkage study localized a susceptibility gene to the region of D2S125-D2S140. The investigators selected a 1.7-Mb region defined by the 1-lod support interval to focus their search. The area contained 7 known genes and 15 expressed sequence tags. The investigators examined 21 SNPs to detect association between type 2 diabetes mellitus and multilocus haplotypes at 3 consecutive SNPs (linkage-dysequilibrium mapping). The haplotype frequency led to the discovery of additional SNPs in the region, some of which were associated with diabetes. They then sequenced a 66-kb interval in 10 diabetic persons, which revealed 3 genes, including CAPN10, and 179 polymorphisms. A complex statistical approach was used to implicate a single SNP in CAPN10, namely an intronic G/A polymorphism. The G allele was associated with a risk for diabetes. Altshuler et al21 referred to the result as "guilt by association," emphasizing the association approach in the analysis. The work was difficult and tedious; the authors spent 4 years doing the study. The finding leaves many unanswered questions. For instance, what is the function of CAPN10? How does the mutation work? How generally applicable are the findings? The work is in our view, however, clearly a harbinger of things to come.
Where will SNP analysis carry us? Insight into this question was given by Weiss and Terwilliger,22 who reviewed the approach and posed the question "How many diseases does it take to map a gene with SNPs?" They provide a critical analysis and outlined the mathematical background of what lies ahead. The contents of their discussion is beyond the scope of this editorial or the talents of this editorial’s authors. Suffice it to say that students of SNP analyses will have to master much material before additional genes for complex diseases can be cloned. The authors couch their discussion in terms of William Faulkner’s The Sound and the Fury (1929): "They all talked at once, their voices insistent and contradictory." Prophetical and poignant, but perhaps what we need now is a discussion along the lines of Berthold Brecht’s inspiring drama Mother Courage (1930): "I translate the Latin of their corrupt preachers into plain language, whereby it is revealed."
Footnotes
The opinions expressed in this editorial are not necessarily those of the editor or of the American Heart Association.
References
1.
Rankinen T, An P, Rice T, Sun G, Chagnon YC, Gagnon J, Leon ASS, Skinner JS, Wilmore JH, Rao DC, Bouchard C. Genomic scan for exercise blood pressure in the HERITAGE Family Study. Hypertension. 2000; 38: 30–37.
2.
Kreutz R, Struk B, Stock P, Hubner N, Ganten D, Lindpaintner K. Evidence for primary genetic determination of heart rate regulation: chromosomal mapping of a genetic locus in the rat. Circulation. 1997; 96: 1078–1081.
3. Lifton RP. Molecular genetics of human blood pressure variation. Science. 1996; 272: 676–680.[Abstract]
4.
Rice T, Rankinen T, Province MA, Chagnon YC, Pérusse L, Borecki IB, Bouchard C. Genome-wide linkage analysis of systolic and diastolic blood pressure: the Québec family study. Circulation. 2000; 102: 1956–1963.
5.
Pankow JS, Rose KM, Oberman A, Hunt SC, Atwood LD, Djoussé L, Province MA, Rao DC. Possible locus on chromosome 18q influencing postural systolic blood pressure changes. Hypertension. 2000; 36: 471–476.
6. DeStefano AL, Baldwin CT, Burzstyn M, Gavras I, Handy DE, Joost O, Martel T, Nicolaou M, Schwartz F, Streeten DH, Farrer LA, Gavras H. Autosomal dominant orthostatic hypotensive disorder maps to chromosome 18q. Am J Hum Genet. 1998; 63: 1425–1430.[Medline] [Order article via Infotrieve]
7.
Levy D, DeStefano AL, Larson MG, O’Donnell CJ, Lifton RP, Gavras H, Cupples LA, Myers RH. Evidence for a gene influencing blood pressure on chromosome 17: genome scan linkage results for longitudinal blood pressure phenotypes in aubjects from the Framingham Heart Study. Hypertension. 2000; 36: 477–483.
8.
Julier C, Delepine M, Keavney B, Terwilliger J, Davis S, Weeks DE, Bui T, Jeunemaitre X, Velho G, Froguel P, Ratcliffe P, Corvol P, Soubrier F, Lathrop GM. Genetic susceptibility for human familial essential hypertension in a region of homology with blood pressure linkage on rat chromosome 10. Hum Mol Genet. 1997; 6: 2077–2085.
9.
Krushkal J, Ferrell R, Mockrin SC, Turner ST, Sing CF, Boerwinkle E. Genome-wide linkage analysis of systolic blood pressure using highly discordant siblings. Circulation. 1999; 99: 1407–1410.
10. Xu X, Rogus JJ, Terwedow HA, Yang J, Wang Z, Chen C, Niu T, Wang Bl, Xu H, Weiss X, Schork NJ, Fang Z. An extreme-sib-pair genome scan for genes regulating blood pressure. Am J Hum Genet. 1999; 64: 1694–1701.[Medline] [Order article via Infotrieve]
11. Xu X, Yang J, Rogus J, Chen C, Schork N, Xu X. Mapping of a blood pressure quantitative trait locus to chromosome 15q in a Chinese population. Hum Mol Genet. 1999; 13: 2551–2555.
12.
Nagy Z, Busjahn A, Bahring S, Faulhaber HD, Gohlke HR, Knoblauch H, Rosenthal M, Muller-Myhsok B, Schuster H, Luft FC. Quantitative trait loci for blood pressure exist near the IGF-1, the Liddle syndrome, the angiotensin II-receptor gene and the renin loci in man. J Am Soc Nephrol. 1999; 10: 1709–1716.
13. Fulker DW, Cherny SS, Sham PC, Hewitt JK. Combined linkage and association sib-pair analysis for quantitative traits. Am J Hum Genet. 1999; 64: 259–267.[Medline] [Order article via Infotrieve]
14.
Boerwinkle E, Hixson JE, Hanis CL. Peeking under the peaks: following up on genome-wide linkage analysis. Circulation. 2000; 102: 1877–1878.
15.
Corder EH, Saunders AM, Strittmatter WJ, Schmechel DE, Gaskell PC, Small GW, Roses AD, Haines JL, Pericak-Vance MA. Gene dose of apolipoprotein E type 4 allele and the risk of Alzheimer’s disease in late onset families. Science. 1993; 261: 921–923.
16. Martin ER, Gilbert JR, Lai EH, Riley J, Rogala AR, Slotterbeck BD, Sipe CA, Grubber JM, Warren LL, Conneally PM, Saunders AM, Schmechel DE, Purvis I, Pericak-Vance MA, Roses AD, Vance JM. Analysis of association at single nucleotide polymorphisms in the APOE region. Genomics. 2000; 63: 7–12.[Medline] [Order article via Infotrieve]
17. Aitman TJ, Glazier AM, Wallace CA, Cooper LD, Norsworthy PJ, Wahid FN, Al-Majali KM, Trembling PM, Mann CJ, Shoulders CC, Graf D, St Lezin E, Kurtz TW, Kren V, Pravenec M, Ibrahimi A, Abumrad NA, Stanton LW, Scott J. Identification of Cd36 (Fat) as an insulin-resistance gene causing defective fatty acid and glucose metabolism in hypertensive rats. Nat Genet. 1999; 21: 76–83.[Medline] [Order article via Infotrieve]
18. Pravenec M, Zidek V, Simakova M, Kren V, Krenova D, Horky K, Jachymova M, Mikova B, Kazdova L, Aitman TJ, Churchill PC, Webb RC, Hingarh NH, Yang Y, Wang JM, Lezin EM, Kurtz TW. Genetics of Cd36 and the clustering of multiple cardiovascular risk factors in spontaneous hypertension. J Clin Invest. 1999; 103: 1651–1657.[Medline] [Order article via Infotrieve]
19. Horikawa Y, Oda N, Cox NJ, Li X, Orho-Melander M, Hara M, Hinokio Y, Lindner TH, Mashima H, Schwarz PEH, del Bosque-Plata L, Horikawa Y, Oda Y, Yoshiuchi I, Colilla S, Polonsky KS, Wie S, Concannon P, Iwasaki N, Schulze J, Baier LJ, Bogardus C, Groop L, Boerwinkle E, Hanis CL, Bell GI. Genetic variation in the gene encoding calpain-10 is associated with type 2 diabetes mellitus. Nat Genet. 2000; 36: 163–175.
20. Hanis CL, Boerwinkle E, Chakraborty R, Ellsworth DL, Concannon P, Stirling B, Morrison VA, Wapelhorst B, Spielman RS, Gogolin-Ewens KJ, Shepard JM, Williams SR, Risch N, Hinds D, Iwasaki N, Ogata M, Omori Y, Petzold C, Rietzch H, Schroder HE, Schulze J, Cox NJ, Menzel S, Boriraj VV, Chen X, Lim LR, Lindner T, Mereu LE, Wang Y-Q, Xiang K, Yamagata K, Yang Y, Bell GI. A genome-wide search for human non–insulin-dependent (type 2) diabetes genes reveals a major susceptibility locus on chromosome 2. Nat Genet. 1996; 13: 161–166.[Medline] [Order article via Infotrieve]
21. Altshuler D, Daly M, Kruglyak L. Guilt by association. Nat Genet. 2000; 26: 135–137.[Medline] [Order article via Infotrieve]
22. Weiss KM, Terwilliger JD. How many diseases does it take to map a gene with SNPs? Nat Genet. 2000; 26: 151–157.[Medline] [Order article via Infotrieve]
Related Article:
Hypertension 2001 38: 30-37.
| |||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||